1. Field of Invention
The present invention relates to a DC-DC converter, and more particularly to a fuseless DC-DC converter which can protect a circuit from overcurrent without involvement of a protective fuse.
2. Related Art
A 12-volt DC power source voltage is usually used as a DC source voltage in a vehicle. However, a load to be used in a vehicle is not limited to a load to be used at a 12 volts DC. For example, a load to be used at 42 volts DC is provided in a vehicle in Europe. In general, after a DC 42-volt source voltage has been lowered to a DC 12-volt source voltage to be used, the DC 12-volt source voltage is supplied to a load to be used at 12 volts DC. The DC 42-volt source voltage cannot be lowered, in unmodified form, to 12 volts DC. For this reason, there has been employed a DC-DC converter, wherein the DC voltage is converted into an AC voltage, the AC voltage is lowered to a desired voltage, and the thus-lowered AC voltage is converted into a desired DC voltage.
A known DC-DC converter has a circuit configuration such as that shown in FIG. 6. A DC power supply (i.e., a battery) is connected to a power MOSFET 11 and a power MOSFET 13 by way of a fuse 10, and the source of the power MOSFET 11 is connected to the drain of a MOSFET 12. Further, the source of the MOSFET 12 is connected to one end of a resistor R10. The remaining end of the resistor R10 is grounded. The source of the power MOSFET 13 is connected to the drain of a MOSFET 14, and the source of the MOSFET 14 is connected to one end of a resistor R11. The remaining end of the resistor R11 is grounded. The power MOSFET 11 and the power MOSFET 13 constitute a higher-potential side of the DC-DC converter.
A primary coil 21 is connected to points between four terminals, one terminal belonging to each of the power MOSFET 11, the MOSFET 12, the power MOSFET 13, and the MOSFET 14; specifically, the primary coil 21 is connected across a junction G between the power MOSFET 11 and the MOSFET 12 and a junction H between the power MOSFET 13 and the MOSFET 14. A secondary coil 22 is disposed so as to oppose the primary coil 21. The turns ratio between the primary coil 21 and the secondary coil 22 is determined in accordance with a target voltage to which the source voltage is to be lowered. When an electric current flows through the primary coil 21, a lower voltage determined by the turns ratio develops in the secondary coil 22.
A driver circuit 15 is connected to the gate of the power MOSFET 11, and the power MOSFET 11 is controlled so as to become active or inactive in response to a gate signal output from the driver circuit 15. The driver circuit 15 is connected to a charge pump circuit 16. The charge pump circuit 16 is made of; for example, a voltage-multiplication capacitor which is constituted by means of stacking capacitors into a plurality of layers, and boosts a source voltage of 12V supplied from the battery to a higher voltage (for example, 22V) and supplies the thus-boosted voltage to the driver circuit 15.
A driver circuit 17 is connected to the gate of the power MOSFET 13, and the power MOSFET 13 is controlled so as to become active or inactive in accordance with a gate signal output from the driver circuit 17. The driver circuit 17 is connected to a charge pump circuit 18. The charge pump circuit 18 is identical with the charge pump circuit 16 in terms of configuration and function.
A driver circuit 19 is connected to the gate of the MOSFET 12, and the MOSFET 12 is controlled so as to become active and inactive in response to a gate signal output from the driver circuit 19. Further, a driver circuit 20 is connected to the gate of the MOSFET 14, and the MOSFET 14 is controlled so as to become active or inactive in response to a gate signal output from the driver circuit 20.
In the DC-DC converter having the previously-described circuit configuration, in a case where the power MOSFET 11, the MOSFET 12, the power MOSFET 13, and the MOSFET 14 are inactive and where the power MOSFET 11 and the MOSFET 14 are simultaneously turned on in response to the gate signals output from the driver circuits 15 and 20, a DC current flows from the battery VB and through the primary coil 21 in the direction designated by arrow C, by way of the drain and source of the power MOSFET 11. The DC current flows to the ground by way of the drain and source of the MOSFET 14 and the resistor R11. As a result of the power MOSFET 11 and the MOSFET 14 being turned on, a half-wave of an AC current (for example, a positive half-wave) is formed; specifically, a DC current whose voltage corresponds to a boosted voltage determined by the turns ratio (i.e., the remaining side of the half-wave; for example, a negative half-wave) arises in the secondary coil 22.
After the power MOSFET 11 and the MOSFET 14 have been activated for a predetermined period of time, the driver circuit 15 deactivates the power MOSFET 11, and the driver circuit 20 deactivates the MOSFET 14. Simultaneously, the MOSFET 12 and the power MOSFET 13 are turned on in response to the corresponding gate signals output from the driver circuit 17 and the driver circuit 19. When the MOSFET 12 and the power MOSFET 13 are turned on, a DC current flows from the battery VB and through the primary coil 21 in the direction designated by arrow D, by way of the source and drain of the power MOSFET 13 (i.e., in the direction opposite that in which the DC current flows when the power MOSFET 11 and the MOSFET 14 are turned on). The DC current flows to the ground by way of the drain and source of the MOSFET 12 and the resistor R10. As a result of the MOSFET 12 and the power MOSFET 13 being turned on, the DC current, which flows in the direction opposite that in which the DC current flows when the power MOSFET 11 and the MOSFET 14 are turned on, induces in the secondary coil 22 a DC current whose voltage corresponds to a lowered voltage determined by the turns ratio (i.e., the remaining half-wave; for example, a negative half-wave). The DC current is converted into an AC current by means of successive occurrence of two types of induced currents (i.e., two types of half-waves).
After the MOSFET 12 and the power MOSFET 13 have been activated for a predetermined period of time, the power MOSFET 11 and the MOSFET 14 are activated for a predetermined period of time. As mentioned above, the power MOSFETs 11 and 14 and the power MOSFETs 12 and 13 are turned on alternately, and a lowered AC current is output from the secondary coil 22. The AC current arising in the secondary coil 22 in the manner mentioned previously is subjected to half-wave rectification (rectification of a positive half-wave) by a half-wave rectification circuit 23. The thus-rectified current is smoothed by a smoothing circuit 24, thereby producing a DC voltage which has been lowered by a predetermined level.
The resistor R10 is for sensing an electric current. In the event that a short circuit or a like failure arising in the secondary circuit is detected as a result of monitoring a potential difference across the resistor R10, the driver circuit 19 is activated to interrupt the MOSFET 12. Similarly, the resistor R11 is for sensing an electric current. In the event that a short circuit or a like failure arising in the secondary circuit is found as a result of monitoring a potential difference across the resistor R11, the driver circuit 20 is activated to interrupt the MOSFET 14.
In the event that a large current develops as a result of a short circuit or a like failure arising in the primary circuit, the fuse 10 is heated when the large current flows through the primary circuit. If an electric current of a predetermined value or higher flows through the primary circuit, the fuse 10 is melted, thereby interrupting the power supply to the primary circuit so as to protect the primary circuit.
As mentioned above, in the known DC-DC converter, in the event that a large current flows through a circuit for reasons of a short circuit, a fuse is melted, thereby interrupting power supply to the circuit. If a large current flows through a circuit for any reason and the fuse is melted, power supply is not supplied to the circuit until the melted fuse is replaced by a new one. Replacing a fuse involves maintenance.
The known DC-DC converter uses a fuse for protecting a circuit. The rating of the fuse is determined by the current designed to flow through the circuit. The diameter of a wire harness of the fuse must be determined in accordance with the rating of the fuse, thereby posing a difficulty in making the wire harness compact.
The known DC-DC converter uses a fuse for protecting a circuit, and the fuse is melted when a large current flows through the fuse. Even when a large current temporarily flows through a circuit, which would be caused by an incomplete short circuit (which would also be hereinafter referred to as a xe2x80x9crare short circuitxe2x80x9d) and not by a complete short circuit (which would also be hereinafter referred to as a xe2x80x9cdead short circuitxe2x80x9d) and would not require interrupting power supply to a circuit, the fuse is melted, thus making detection of an anomalous short circuit impossible.
Since the known DC-DC converter utilizes melting action of a fuse for protecting a circuit, there is a necessity for using, as a wire of a circuit constituting a DC-DC converter, a wire harness whose diameter is sufficient to withstand the current which flows through a circuit in the event of occurrence of a dead short circuit, thus posing a difficulty in rendering the wire harness compact.
The present invention is aimed at eliminating a necessity for maintenance for reactivating a DC-DC converter even when circuit protection is effected at the time of flow of a large current due to a short circuit or a like failure, reducing the diameter of a wire harness and making the wire harness compact, and readily detecting occurrence of a rare short circuit.
Accordingly, the present invention provides a fuseless DC-DC converter which includes a plurality of parallel-connected FETs, is repeatedly turned on and off by alternate activation/deactivation of higher-level FETs and activation/deactivation of lower-level FETs, to thereby induce an AC current from a DC current in midpoints between the higher-level FETs and the lower-level FETs, boosts or lowers the AC current to a predetermined voltage, and converts the AC current into a DC current, to thereby produce a DC power supply whose voltage is boosted or lowered with reference to a source voltage, wherein
one of the parallel-connected FETs is embodied by a power supply controller, the power supply controller comprising:
a load circuit formed by placing a first overheat self-interruption-type semiconductor switch in series between a DC power supply and a load;
a second overheat-self-interruption-type semiconductor switch connected in parallel with the first overheat-self-interruption-type semiconductor switch;
a reference circuit whose one end is connected to the source of the second overheat-self-interruption-type semiconductor switch and whose other end is grounded and which induces, across the drain and source of the second overheat-self-interruption-type semiconductor switch, the same voltage as that arising across the drain and source of the first overheat-self-interruption-type semiconductor switch when a constant load current flows through the first overheat-self-interruption-type semiconductor switch; and
a comparator circuit which compares the source voltage of the first overheat-self-interruption-type semiconductor switch with a reference voltage applied to the source of the second overheat-self-interruption-type semiconductor switch, and
the power supply controller deactivating the first overheat-self-interruption-type semiconductor switch when, on the basis of the result of the comparison performed by the comparator circuit, a current of a predetermined value or greater is determined to have flowed through the first overheat-self-interruption-type semiconductor switch; controlling activation or deactivation of the first overheat-self-interruption-type semiconductor switch under predetermined conditions and at a predetermined duty cycle; determining that an anomaly, such as a short circuit, has arisen in the load circuit, when the activation and deactivation of the first overheat-self-interruption-type semiconductor switch has continued for a predetermined period of time; and interrupting the first overheat-self-interruption-type semiconductor switch, to thereby suspend power supply to the load.
By means of the above-described configuration of the present invention, a circuit can be protected from a large current, which would otherwise be caused by a short circuit or a like failure, without use of a fuse. Even if a circuit is protected from a large current at the time of a short circuit, reactivation of the circuit does not involve maintenance. The diameter of a wire harness is reduced, thus saving the weight of the wire hardness.
Preferably, the predetermined conditions under which the first overheat-self-interruption-type semiconductor switch is controlled to be activated or deactivated at a predetermined duty cycle are such that the first overheat-self-interruption-type semiconductor switch is deactivated when the voltage across the drain and source of the first overheat-self-interruption-type semiconductor switch has become smaller than a threshold voltage set to 60% to 80% the source voltage and when the source voltage of the first overheat-self-interruption-type semiconductor switch has become higher than the source voltage of the second overheat-self-interruption-type semiconductor switch; and such that the first overheat-self-interruption-type semiconductor switch is deactivated when the voltage across the drain and source of the first overheat-self-interruption-type semiconductor switch has become higher than a threshold voltage set to 60% to 80% the source voltage.
By means of the foregoing configuration of the present invention, a circuit can be protected from a large current, which would otherwise be caused by a short circuit or a like failure, without use of a fuse. Even if a circuit is protected from a large current at the time of a short circuit, reactivation of the circuit does not involve maintenance. The diameter of a wire harness is reduced, thus saving the weight of the wire hardness. Further, the present invention enables not detection of complete short circuit (i.e., a dead short) but facilitated detection of an incomplete short circuit (i.e., a rare short).
Preferably, the power supply controller is additionally provided with a forceful driver circuit which forcefully activates the first overheat-self-interruption-type semiconductor switch, by application, to the comparator circuit, of a partial voltage which is obtained by division of the source voltage, when a potential difference across the drain and source of the first overheat-self-interruption-type semiconductor switch is increased by an internal resistor of the first overheat-self-interruption-type semiconductor switch, after the first overheat-self-interruption-type semiconductor switch has been deactivated on the basis of an output which is issued by the comparator circuit upon detection of an overcurrent due to an anomaly, such as a short circuit in the load circuit.
By means of the foregoing configuration of the present invention, a circuit can be protected from a large current, which would otherwise be caused by a short circuit or a like failure, without use of a fuse. Even if a circuit is protected from a large current at the time of a short circuit, reactivation of the circuit does not involve maintenance. The diameter of a wire harness is reduced, thus saving the weight of the wire hardness. Further, the present invention enables determination as to whether or not the flow of a large current is ascribable to a complete short circuit (i.e., a dead short) or another, temporary reason.